Newbie Prop Questions

[DYLBOSS]

Sorry for the basic questions, but when I tried searching and all I got was information that is way over my head.

I'm trying to understand how my boat is performing and if it is normal. I really don't understand what pitch of a prop means, and also how a slight nick in a prop would affect performance. I have a very small nick on one of the blades. Also, what difference would a bigger or smaller prop make overall? Guess I'm just trying to understand prop basics.

This is what I have:

1996 Rinker Captiva 212 Cuddy
8' Beam
Deep-V
Dry Weight 2750
Tank - 40 Gallon
5.7 Merc 260HP w/Alpha One outdrive


Thanks!

 

[TenBarge]

Re: Newbie Prop Questions

I am a newbie to this as well, and am cleaning/summerizing my first boat and was curious about pitch/trim of a prop as well.

 

[hwsiii]

Re: Newbie Prop Questions

Dyl, pith is the theoretial movement of a prop through the water in one revolution. A 20" pith prop, if it has zero prop slip should move the boat forward 20" every time the prop turns one revolution but beause water is a liquid and not a solid it has what is alled prop slip. Think of a srew turning in a piee of wood that has a 20" pith every time the srew turns one revolution it will go forward 20". But in boat props it ant move forward 20" in eah revolution beause it is in a liquid and it has a ertain amount of slip, that differene in how far the prop should move forward and how far it Atually moves forward is alled prop slip.
I will be glad to help you if will fill out this form.

Iboats Boat and Motor Info
1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the recommended HP for your boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make. manufacturer and model of motor
7. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing and turn the motor until the tape lines up with each other, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs
12. Make, model, diameter, pitch and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. If you do not have a tach you can buy a Tiny Tach for $ 50
14. Are you at sea level or a higher elevation, give us the elevation in feet
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark, all of the foregoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations
19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.


H

 

[DYLBOSS]

Re: Newbie Prop Questions

Wow. A lot of this stuff is over my head. I guess I'll try and gather as much info as I can.

Thanks!

 

[ramster]

Re: Newbie Prop Questions

What is your top speed at what rpm? That will tell you if you are close to where you should be in terms of performance. I think that that boat should run about 50 mph@4600rpm or somewhere close to that. Are you familiar with how to trim the drive up to get max rpm?

 

[sho3boater]

Re: Newbie Prop Questions

Pitch ends up being the gearing of the motor to the water. More pitch you go faster or the motor runs slower if it can't provide enough power. If the motor runs too slow you get less power from it. So, you pitch the prop to run where the engine makes the most power, that should be on the motor or look it up. It might be 5,000rpm at peak HP for example, and a perfect prop might run 5200 on a perfect cool day with nothing but you in the boat and a little chop to loosen the hull. Even with people and gear you should be close to 5K with that prop. The boat will go the fastest that way (most of the time, but also it will run properly at the spec rpm and last longer). If you always have gear in the boat, then run less pitch to get proper rpm loaded. Ski boats often do for more towing power, but you have to be careful to not over rev them running top speeds.

You trim up at planing speeds to lift the bow and unwet some of the hull, less drag makes for more speed and better mpg. In rough conditions you must trim down to bridge the waves with the full length of the hull or the ride gets nasty and eventually dangerous.